Besides GCC there's also POC, but I don't know if David Stes still develops it, and it is not compatible with NeXT's extensions to Objective-C AFAIR.

Finding anything resembling a decent IDE for fringe languages is never easy. If you want to dick around with something you could always download the trial of Dolphin Smalltalk--it won't do much to broaden your horizons to the Mac world, but it would hardly be the worst use of curiosity and spare time.